update lz4_flex crate to v0.11

[nestor-sk]

This PR updates the lz4_flex rust crate to the latest version (0.11), update tests to match the changes in behavior since 0.9 and adds a little example in the readme on how to use the recent frame format functions

Details:

• In native/nimblelz4/src/lib.rs added block:: for the compress/decompress functions, because the export of those is deprecated in 0.11
• Modified empty frame encoding test to account for https://github.com/PSeitz/lz4_flex/pull/120 part of 0.11
• Modified incorrect size in block decoding test to account for https://github.com/PSeitz/lz4_flex/pull/78 part of 0.11

[whatyouhide]

Nice, fantastic! Thanks for the contribution. Can you run the formatter? CI is failing due to that.